The tournament hosts' place in the competition hangs in the balance following defeat to Wales last time out and they face a must-win clash in Group A.
Rowntree warned his players that they must concentrate solely on their jobs in key moments against the Wallabies at Twickenham Stadium.
"It was always going to be a big game, it's a pool game against Australia - a very good team - one of the best in the world," he said.
"We trained exceptionally well this week. Last week (against Wales) has hardened us, there's been a real focus about what we've done this week, a real focus. We trained, but we've got to have some composure if we're going beat one of the best teams in the world."
Billy Vunipola has been replaced by Nick Easter in the squad after the number eight was ruled out of contention with knee ligament damage.
Rowntree believes Stuart Lancaster’s decision to call up the 37-year-old has been positive.
"Nick's certainly been involved in a lot of those furnaces (games) as a player and so have I. Because these days it's all about what you do under pressure. And that's what we spoke about; extreme composure, sticking to the plan, sticking to doing your role. It's as boring as that, I'm afraid," he said.
"He was very unlucky not to make the original cut. Straight back in, he's involved in the matchday squad. We like a natural eight in a matchday squad on the bench. But saying that, he covers a couple of other positions as well and his experience will be needed tomorrow night. He's been great this week with the lads, you know all summer he's been good with the likes of Joe (Launchbury) and the young guys."